The present invention relates to a demodulation method, and demodulation apparatus for reproducing data that can reduce reproduced data errors caused by a received signal level variation that is incurred by switchover of an antenna of a transmission side or other causes in transmission of radio data using a radio wave from a mobile body such as in a helicopter or a vehicle, or a stationary body to a mobile body or a stationary body.
In the case where data is radio-transmitted from a transmission apparatus (hereafter referred to as transmission side) mounted on a mobile body such as a helicopter or fixed on the ground to a reception apparatus (hereafter referred to as reception side) installed on a stationary on the ground or mounted on a mobile, a transmission antenna is attached to the transmission side. According to the relative position relation between the transmission antenna of a mobile body such as a helicopter and a reception antenna of the reception side, which receives a radio wave sent from the transmission antenna, and the travel direction of the mobile, a part of the mobile body obstructs the transmitted radio wave in some cases. Typically, therefore, a plurality of transmission antennas are attached to the left and right sides or front and rear portions of the mobile body of the transmission side. On the basis of the angle formed by the transmission antenna of the mobile body and the reception antenna, a transmission antenna having visibility with respect to the reception antenna is selected from among a plurality of transmission antennas and switchover thereto is conducted. A radio wave for data signal transmission is sent from the selected transmission antenna.
As a concrete example of the foregoing radio transmission system, such an example that the transmission side is a traveling helicopter and the reception side is installed and fixed on the ground will now be described by referring to FIG. 7.
Numeral 108 denotes a reception antenna C connected to the reception side installed on the ground. Numeral 109 denotes a helicopter. Numerals 106 and 107 denote two transmission antennas A and B attached to the main body of the helicopter 109. Characters La and Lb denote a radio propagation distance between the transmission antenna A 106 and the reception antenna C 108 and a radio propagation distance between the transmission antenna B 107 and the reception antenna C 108, respectively. Radio wave emission from the helicopter 109 is conducted from either the transmission antenna A 106 or the transmission antenna B 107 on the basis of the relative position relation between the transmission antennas and the reception antenna as described later.
As for the selection rule of the transmission antenna, the antenna B 107 is used while the reception antenna C 108 is located on the left side (bottom side in FIG. 7) with respect to the flight direction D of the helicopter 109. The antenna A 106 is used while the reception antenna C 108 is located on the right side (top side in FIG. 7), the antenna A 106 is used.
Therefore, means for conducting antenna switchover described later is provided on the transmission side. An angle formed by the direction of the reception antenna C 108 viewed from the helicopter 109 with respect to a flight direction D is denoted by θ. When a predetermined change has occurred in the angle θ, switching between the transmission antennas A and B (106 and 107) is conducted accordingly. The sign (polarity) of the angle θ in the direction indicated by the arrow (counter clockwise) shown in FIG. 7 is positive (+), and negative (−) in the opposite direction (clockwise). When the sign of the angle θ changes from positive to negative, or changes from negative to positive due to a change of the flight direction D of the helicopter 109 and the absolute value of the angle θ at that time has become at least a predetermined value φ in the range of, for example, one to three degrees, the transmission antenna is switched over.
It is now assumed that at first a radio wave is emitted from the transmission antenna B 107 and received by the reception antenna C 108 and data transmission is conducted. And it is assumed that the sign of the angle θ formed by the flight direction D of the helicopter 109 and the reception antenna C 108 changes due to a change of the flight direction D of the helicopter 109 as shown in FIG. 7 and the absolute value has become equal or more than the predetermined angle φ. Thereupon, data transmission conducted heretofore by a radio wave emitted from the transmission antenna B 107 is switched over to a radio wave emitted from the transmission antenna A 106 having clearer sight with respect to the reception antenna C 108.
Even if the sign of the angle θ formed by the flight direction of the helicopter 109 and the reception antenna C 108 does not change while its absolute value becomes large, thereafter emission of the radio wave from the transmission antenna A 106 is continued.
If switching between the two transmission antennas A and B (106 and 107) is conducted, however, the following problem occurs. In the case where transmission is conducted by using a radio wave having a carrier frequency of, for example, 7 GHz, one wavelength is approximately 4.3 cm (=the velocity of light 3.0×1010 (cm/second)× 1/7 GHz). It is now assumed that the distance between two transmission antennas A and B (106 and 107) attached to the helicopter 109 is 400 cm. In the case where the angle θ is 1 degree, the distance difference (=La−Lb) between the distances from two transmission antennas A and B (106 and 107) to the reception antenna C 108 on the ground is approximately 7 cm. The shift of the carrier phase of the radio wave becomes 360 degrees×7 cm÷4.3 cm=586 degrees, i.e. 226 (=586−360) degrees. If antenna switching is conducted at the angle θ=1 degree, then the phase of the carrier of the radio wave the signal on which is received on the ground thus changes by 226 degrees.
In this way, the carrier phase of the received radio wave suddenly changes greatly due to the transmission antenna switching. Or as a matter of fact, there is a signal level difference depending upon the circuit ranging from the antenna switchover switch to radio wave sending via the transmission antenna A 106 or the transmission antenna B 107, the transmission antenna, the cable length, and so on. Accordingly, a transmitted signal level difference between the transmission antennas A and B (106 and 107) is caused by the transmission antenna switchover. This results in a problem that data errors occur on the reception side.
In the case of a multi-level amplitude modulation scheme, the decistion margin for the amplitude becomes very small especially as the number of multiple levels increases. For example, a signal constellation of the 64 QAM (Quadrature Amplitude Modulation) is shown in FIG. 9. In a decision unit of the demodulation apparatus, a decision boundary as represented by a dotted line is provided with respect to a signal position of a received signal (black dot in FIG. 9). Even if there are influence of noise, an equalization error, and a received signal level variation, a signal point received from the transmission side can be estimated and decision correctly and received data can be reproduced unless the received signal exceeds the decision boundary.
As an example, a signal constellation in the case where the received signal level has suddenly dropped by approximately 1 dB is shown in FIG. 10. When the amplitude of the signal position of the received signal (black dots in FIG. 10) becomes small, the signal position approaches the origin, and gets out of the reference point located in the center of the decision boundary area (area surrounded by dotted lines). Therefore, the margin from the decision boundary is decreased. In this situation, if there is noise on the transmission path, an equalization error, a received signal level variation, and so on, then the received signal exceeds the decision boundary, and even when the noise is added to the signal or the transmission data cannot be reproduced correctly, resulting in errors of received data. In addition, in the case where a sudden change of the received signal level generated at the time of transmission antenna switchover is great, by only such change of signal level the decision boundary and errors are caused in received data.